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8/7/2019 THE ROLE OF SUBMERSED AQUATIC VEGETATIONAS HABITAT FOR FISH IN MINNESOTA LAKES,INCLUDING THE IMPLICA
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Minnesota Department of Natural ResourcesSpecial Publication 160, 2004
THE ROLE OF SUBMERSED AQUATIC VEGETATION
AS HABITAT FOR FISH IN MINNESOTA LAKES,INCLUDING THE IMPLICATIONS OF NON-NATIVE PLANT INVASIONS
AND THEIR MANAGEMENT
Ray D. Valley, Timothy K. Cross, and Paul Radomski.
Minnesota Department of Natural Resources
Division of Fish and Wildlife500 Lafayette Road
St. Paul, MN 55155-4020
Executive Summary
This review updates the Division of Fish and Wildlifes understanding of the role of
submersed aquatic vegetation (SAV) in providing fish habitat in Minnesota lakes. Below, are
several generalizations and recommended approaches for aquatic plant management.
1. Many fish, such as sunfish, largemouth bass, northern pike, and muskellunge, de-
pend on SAV for food and shelter. Nongame fish such as darters, minnows, and
killifishes depend primarily on nearshore emergent and submersed vegetation.
2. The presence of SAV tends to promote higher water clarity.3. Black bullhead and common carp often dominate turbid lakes with little to no SAV.
Carp are an invasive non-native species that contributes to the loss of native SAV by
dislodging rooted plants and resuspending sediments.
4. Generally, conditions for game fish deteriorate when the percentage of a basin that is
covered with SAV falls below 10% or exceeds 60%. This range does not consider
basin morphometry (i.e., shallow versus deep) which ultimately controls how much
vegetation naturally grows within a lake.
5. Studies show native plants provide higher quality habitat for desirable fish than in-
vasive non-native plants such as curly-leaf pondweed or Eurasian watermilfoil.
However, these non-native plants provide better habitat than little or no SAV.
6. Minnesota lakes infested with curly-leaf pondweed or Eurasian watermilfoil have
not seen large declines in game fish populations.7. Lake productivity and initial plant conditions appear to greatly affect selective
whole-lake herbicides (such as fluridone) effect on fish habitat. Whole-lake studies
in infested, moderately-productive (mesotrophic) Michigan lakes with abundant na-
tive plants, showed neutral to positive effects of fluridone on fish habitat.13. Fluridone applications in infested productive (eutrophic) Minnesota lakes with low
cover of native SAV can have dramatic negative effects on SAV habitats, water clar-
ity, and fish communities.
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14. Aquatic plant management policies should reflect a precautionary approach where it
is understood that any alteration to SAV will invariably have some effect on a lakes
fish community. Therefore, policies should ostensibly be conservative with the in-
tent to minimize habitat degradation.
15. Limiting the cumulative amount of SAV removal may be the most prudent approach
towards precautionary management. However, thresholds should be dependent on
lake type. The current 15% rule (maximum treatment area within the 15 foot depthzone) for chemicals and 50% rule for mechanical harvesting may be reasonable for
some lakes (e.g., small eutrophic lakes); stricter thresholds may be needed for others(e.g., soft water lakes, large or deep lakes).
16. Overall, whole-lake aquatic plant treatment is risky. Significant biological risks as-
sociated with large-scale manipulations include excessive removal of fish habitat
and thus decline of fish populations, loss of sensitive plant species, declines in water
clarity and potential long-term cumulative effects of multiple treatments, sinceeradication of non-native plant species is highly unlikely.
17. Vegetated, nearshore habitat is critical for fish recruitment. Any removal should be
viewed as habitat loss, and efforts should be made to minimize this loss. It follows
that 100 feet of removal is worse than 50 feet of removal even if the removal is of a
non-native species.18. Mechanical harvesting may be the best alternative for managing nuisance surface
growth of vegetation. Although this requires perpetual maintenance, harvested boat
lanes through surface-growing vegetation represents a positive benefit for recrea-
tional access and fish habitat (harvested strips of SAV increases edge and may
benefit game species).
Introduction
The role of submersed aquatic vegeta-
tion (SAV) in structuring lake ecosystems, and
in particular fish communities, has been the
source of much research during the last 25
years. Several published papers comprehen-
sively review these roles and demonstrate that
the relationship between SAV and fish is
highly complex and variable (Dibble et al.
1996; Weaver et al. 1996; Diehl and Kornijw
1998; Petr 2000). Nevertheless, several gen-
eralizations have emerged from this collective
body of research. They are discussed here in
the context of Minnesota lakes.
The relationship of SAV and fish var-
ies among lake types, thus we must first
establish a framework on which these gener-
alizations may be based. The quality of lakes
as habitat for fish, depend on their glacial his-
tory, climate, water chemistry, productivity,
and morphometry (Tonn and Magnuson 1982;
Tonn 1990; Schupp 1992). In Minnesota, we
see a great diversity of lake types, each dis-
playing its own physical and biological
signature (Figure 1, Table 1). Environ-
mental filters, hierarchical in space and time
(i.e, continental regional or watershed lake-type within-lake), determine these sig-natures and constrain fish communities within
a defined range (Tonn 1990). Herein, we fo-
cus primarily on the lake type and within lake
effects of SAV on fish populations. However,
the following generalizations should be inter-
preted as nested within the context of these
larger filters operating on any particular wa-
terbody of interest.
Aquatic plants serve many ecosystem
functions including primary production, stabi-
lizing sediments, maintaining water clarity,
and providing habitat for zooplankton, macro-invertebrates, and numerous fish species
(Carpenter and Lodge 1986; Dibble et al.
1996; Diehl and Kornijw 1998). Many
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L a k e S ize( A c r e s )
< 5 0 0 > 5 0 0
L a k e D e pt h( % < 1 5 f t)
> 8 0 % < 8 0 % > 8 0 %
To ta l A lka lin i ty( m g /L C aC O 3)
> 1 0 0 < 1 0 0 > 1 0 0 < 1 0 0 > 1 0 0 < 1 0 0< 1 0 0
L a k e Ty p e 1 L a k e Ty p e 3
L a k e Ty p e 2 L a k e Ty p e 4
L a k e Ty p e 5
L a k e Ty p e 6
L a k e Ty
Figure 1. Physical classification of Minnesota Lakes using a reduction or lumping of Schupps (1992) lake tion of large and small lakes is arbitrary and relative to the size range of lakes in Minnesota.
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Table 1. Characteristics of the eight distinct types of Minnesota Lakes identified in Figure 1.
aProductivity is based on Trophic State Index (Carlson 1977) and is a combination of water clarity (secchi), phosphorus concentration (totion (Chlorophyll a).
bRelative plant species richness was determined by DNR vegetation survey species lists.
cStatistical analyses (cluster analysis) to determine relative abundance of aquatic vegetation was performed on recent vegetation survMinnesota DNR Fisheries field personnel (P. Radomski, unpublished data).
d
Alternate stable states refers to the dynamic characteristic of shallow lakes whereby abundant vegetation and relatively clear water occsubmersed vegetation occupies another mutually-exclusive state.
e Fish species richness in this lake class depends on the productivity and the SAV richness in the lake; highly productive lakes with speciesspecies richness.
LakeType
Description
Number
of Lakes Location Productivitya
RelativePlant Spe-cies
Richnessb
Relative SAV
Abundancec
AlternateStable
Statesd
1 Small, shallow, softwater
991 N. Central NE
Moderate to high High Low
2 Small, shallow,hard water
408 Central SW high to very high Low Low to High
3 Small, deep, softwater
685 N. Central NE
Moderate High Low
4 Small, deep, hardwater
1,173 Central Moderate to high Moderate Low to High
5 Large, shallow, softwater
34 NE High High Low
6 Large, shallow,hard water
190 Central S.Central
High Low Low to High
7 Large, deep, softwater
102 NE Low to moderate No Data Low
8 Large, deep, hardwater
340 N. Central Moderate to high High High
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species of fish depend on SAV for their sur-
vival. In fact, Dibble et al. (1996) noted that
112 fish species representing 19 families were
found in SAV habitats in the Upper Missis-
sippi River Basin. Studies consistently
demonstrate that fish abundance is greater in
vegetated habitats than in unvegetated habitats(Dibble et al. 1996; Cross and McInerny 2001;
Pratt and Smokorowski 2003). Submersedvegetation attracts an abundance of inverte-
brates that provides prey for many juvenile
game fish and nongame fish species (Keast
1984). This attracts larger, predatory game
fish that patrol the edge of plant beds or waitin open pockets to ambush fish prey (Killgore
et al. 1989). Many fish species depend on
SAV for nest building and spawning. Near-
shore vegetation is particularly important
nursery habitat for young age classes of fishand small nongame fish (Poe et al. 1986;
Bryan and Scarnecchia 1992). Nearshore
vegetation also is critical for many wildlife
species including ducks and wading birds.
Aquatic vegetation is consumed by some ani-
mals while other animals consume the
macroinvertebrates living on SAV.
In Minnesota lakes, SAV is important
to game fish such as sunfish Lepomis spp.,
largemouth bass Micropterus salmoides,northern pike Esox lucius, and muskellunge
Esox maquinongy. These species depend onSAV for spawning, refuge and foraging.
Other species such as smallmouth bass Mi-cropterus dolomieu, yellow perch Perca
flavescens, walleye Sander vitreus, crappie
Pomoxis spp., white bass Morone chrysops,and catfish Ictalurus spp. are adapted to openwater environments, and SAV is of less im-
portance for their populations (Dibble et al.
1996; Cross and McInerny 2001).
Vegetated Minnesota lakes support a
diversity of nongame species including nu-
merous minnow and shiner species (Family:Cyprinidae), yellow bullhead Ameiurus na-
talis, banded killifish Fundulus diaphanus, brook silverside Labidesthes sicculus, and
some darter species Etheostoma spp. In con-trast, black bullhead Ameirus melas and non-
native carp Cyprinus carpio are often associ-ated with turbid lakes with little to no SAV
because they are tolerant of the low oxygen
environments typically associated with these
lakes (Drake and Pereira 2002). Carp directly
affect the abundance of SAV by dislodging
rooted plants while feeding, and reducing op-
portunities for reestablishment by increasing
the sediment and nutrients in the water col-
umn. In doing so, they increase their
competitive advantage over most game fish(Breukelaar et al. 1994; Lammens 1999; Par-
kos et al. 2003).
Given the importance of SAV for fish
populations and productive fisheries, fisheries
managers have sought to identify the level of
aquatic plant abundance that supports high
game fish production, particularly for large-mouth bass and bluegill L. macrochirus.
Crowder and Cooper (1979) developed a
model that suggested that predator feeding
rate, and thus growth and production, should
be highest at intermediate levels of plant abun-dance. At low plant abundances, prey are
scarce because of a lack of predator refuges.
At high plant abundances, visual and swim-
ming barriers created by dense vegetation
reduce the ability of predators to capture fish
prey. Indeed, several experimental studies
have documented a reduction in predator feed-
ing rates as vegetation becomes more dense
(Crowder and Cooper 1982; Savino and Stein
1982; Anderson 1984; Gotceitas and Colgan
1987; Diehl 1988; Valley and Bremigan
2002a).Numerous studies from southern U.S.
lakes and reservoirs have, at least in part, sup-
ported this theory. In general, when
vegetation covers less than 10% of a water-
body (e.g., turbid reservoirs), vegetation-
dependent species are scarce and production
of largemouth bass and bluegill is low
(Durocher et al. 1984; Bettoli et al. 1992; Ma-
ceina 1996; Wrenn et al. 1996, Miranda and
Pugh 1997). When vegetation coverage ex-
ceeds 40-60% of the entire waterbody (i.e.,
shallow vegetated lakes or bays), low feedingrates or poor growth by predators commonly
result (Colle and Shireman 1980; Bettoli et al.
1992; Maceina 1996; Wrenn et al. 1996;
Miranda and Pugh 1997). This suggests thatthe probability of finding a quality largemouth
bass or bluegill population is highest between
10% and 60% total cover of SAV. Less work
has been done in north temperate lakes, never-
theless, Wiley et al. (1984), Trebiz et al.
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(1997), Schneider (2000), and current studies
in Michigan (K. Cheruvelil and N. Nate,
Michigan State University, Department of
Fisheries and Wildlife, unpublished data) to-
gether suggest a preferred range of 18%
40% total cover of SAV.
Pursuit of this elusive optimal per-cent coverage of SAV for littoral fish species
has revealed numerous complexities of lakesthat taken together, question the appropriate-
ness of using a defined percentage in
structuring management decisions (Hoyer and
Canfield 2001). Below, we list important
characteristics and features of lakes that areoverlooked when relating coarse estimates of
SAV cover to fish.
Availability of other critical microhabitatssuch as spawning substrates (Annet et al.
1996; Weaver et al. 1996). Ecological constraints operating at scales
greater than the scale of analysis (Tonn
1990; Hinch 1991; Dibble et al. 1996;
Weaver et al. 1996; Miranda and Dibble
2002).
Lake depth controls the maximum coverof SAV. Naturally, lakes with > 60%
cover are shallow lakes. No published
studies to our knowledge show reducing
vegetation cover in shallow lakes im-
proves fish growth. In deep lakes, percent
whole-lake cover of SAV will never beexcessive (Hoyer and Canfield 2001).
Most published studies have not explored
the effect of a range of SAV cover solely
within the littoral zone.
Lake size, depth, shape, water chemistry,trophic status, substrates (Downing et al.
1990; Downing and Plante 1993; Chick
and McIvor 1994; Hoyer and Canfield
1996; Maceina 1996; Weaver et al. 1996;
Vestergaard and Sand-Jensen 2000)
Aquatic plant architecture and patchiness(Chick and McIvor 1994, 1997; Weaver etal. 1997; Valley and Bremigan 2002a).
For example, 100% cover of a low-
growing macrophyte species such as chara
Chara spp. provides vastly different habi-
tat than 100% cover of a canopy species
such as Eurasian watermilfoil Myriophyl-lum spicatum.
Composition of the food web and interac-tions between littoral and pelagic habitats
(Diehl and Kornijw 1998; Lodge et al.
1988).
Habitats created by plants are affected
by, and nested within, other physical forces
affecting the total habitat heterogeneity of a
particular lake. We use the term heterogene-
ity to reflect what is commonly referred to as
quality. Quality is subjective and difficult
to measure. Heterogeneity simply refers to a
collection of diverse micro-habitats suitablefor a variety of fish species. Factors such as
lake size, shape, depth, substrate composition,
water chemistry, and productivity influence
the distribution of non-macrophyte habitats(e.g., gravel substrates), as well as control thecomposition, abundance, and distribution of
plant species assemblages (Figure 2; Duarte
and Kalff 1990; Nichols 1992; Weaver et al.
1996; Vestergaard and Sand-Jensen 2000).
Ultimately, heterogeneity will be highest in
large lakes with convoluted shorelines and
complex basin morphometry (i.e., variable
slopes, and numerous points, and islands),
high water clarity with moderate nutrient lev-
els, and high substrate diversity (i.e., patches
of clay, muck, sand, and gravel). These condi-tions provide habitats for a diversity of aquatic
plant species, thereby providing habitat for a
diversity of fish species.
It is widely recognized that holding all
the above habitat factors constant, quality fish
habitat is associated with diverse plant com-
munities. Diversity in the architectural growth
form (arrangement of stems and leaves) of
individual plant species and the spatial distri-
bution and species composition of plant beds
creates a patchy littoral landscape. Studies
demonstrate that a diversity of plant types andpatchiness is positively related to fish diversity
(Weaver et al. 1996, 1997; Pratt and
Smokorowski 2003), abundance
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Lake
Productivity
Oligotrophic Mesotrophic Eutrophic Hyper-
eutrophic
Lake
Size
Small Large
Basin Morphology and
Shoreline
Round
Bowl
Shallow Convoluted
depth andshoreline
Substrate
Homogeneous Heterogeneous
Figure 2. Conceptual heterogeneity Equalizer with gray bars demonstrating levels of highest
habitat heterogeneity. Collectively, these characteristics generally lead to diverse macro-
phyte assemblages, thereby providing a feedback to habitat heterogeneity.
(Killgore et al. 1989; Chick and McIvor 1994,
Pratt and Smokorowski 2003) and predator
foraging abilities (Valley and Bremigan
2002a). Multiple age-classes and species of
fish depend on a diversity of habitats for their
spawning, foraging, and refuge needs (Chick
and McIvor 1994; Werner et al. 1977; Annet
et al. 1996; Weaver et al. 1996, 1997).
Patchiness is highly scale-dependent
and is perceived differently by species of fishdepending on their size and home range
(Kotliar and Weins 1990; King 1993; Essing-
ton and Kitchell 1999; Pratt and Smokorowski
2003). Therefore, for small vegetation dwell-
ing species, plant composition at small scales
(1 x 1 x 1 m volume of water close to shore) is
perhaps more important than the spatial ar-
rangement and size of large offshore beds of
vegetation. In contrast, larger, more mobile
species such as adult largemouth bass, north-
ern pike, and muskellunge respond to
patchiness at larger scales, and a lakes patch
mosaic becomes increasingly important to
their success (Cross and McInerny 2001). Di-
verse native plant communities produce
heterogeneity at multiple spatial scales.
SHALLOW LAKES UNIQUE IN THEIR
STRUCTURE AND FUNCTION
In our consideration of the diversity of
Minnesota lakes, shallow lakes (lake types 1,
2, 5, and 6) deserve special attention. Figure 2
suggests, shallow lakes inherently have low
habitat heterogeneity. Environmental factors
such as oxygen depletion in winter further
contributes to poor habitat conditions for fish.
In general, fish communities are depauperate
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in shallow, eutrophic Minnesota lakes (Drake
and Valley in review). Rather, SAV in shal-
low Minnesota lakes serves a greater role in
providing habitat for waterfowl and other wet-
land-associated species.
Hosts of unique biological and physi-
cal processes within shallow lakes affect thecomposition and abundance of macrophytes.
Shallow, eutrophic lakes typically occupy oneof two alternative states: one characterized by
clear water and abundant aquatic vegetation,
or one characterized by turbid water and little
to no vegetation (Blindow et al. 1993; Schef-
fer et al. 1993). High external nutrient loads,internal mixing (i.e., bottom-up forces) or a
high abundance of zooplanktivorous or ben-
thivorous fishes (top-down forces), increase
the probability that phytoplankton and algae
will dominate the water column (Carpenter etal. 1985; Bronmark and Weisner 1992;
Jeppesen et al. 1997). Given low external and
internal loading, or low abundance of plank-
tivorous fish, aquatic plants can colonize
littoral areas and maintain a clear-water state.
Return from a turbid to a clear, macrophyte
dominated state can be difficult to achieve
without substantial intervention. Management
techniques include water level drawdowns,
substantial reductions to nutrient inputs and
control of planktivores through predator stock-
ing or direct removal (e.g., biomanipulation;Hanson and Butler 1990, Moss et al. 1996;
Carpenter and Cottingham 1997; Herwig et al.
2004). Deeper lakes are generally more resis-
tant to a conversion to a turbid state because
the hypolimnion in stratified pelagic zones act
as a sink for nutrients (Carpenter and Cotting-
ham 1997). Nevertheless, deep lakes in
heavily developed or agricultural watersheds
often occupy a turbid-water state due to per-
petually high nutrient loading from the
watershed (Cross and McInerny 2001). Some
of the same techniques used to rehabilitateshallow lakes may lead to improvements in
these deeper lakes.
EFFECTS OF INVASIONS OF NON-
NATIVE PLANTS ON FISH HABITAT
Invasion by non-native species of
submersed aquatic plants may displace native
plant species and reduce the suitability of
habitat for certain species of fish, as well asinterfere with recreation. In Minnesota lakes,
the establishment of non-native invasive spe-
cies of submersed aquatic plants such as
Eurasian watermilfoil and curly-leaf pond-
weed Potamogeton crispus are a concern tolake users, local units of government, and the
Minnesota Department of Natural Resources(DNR).
Eurasian watermilfoilEurasian wa-termilfoil (EWM) was introduced into the U.S.
in the 1940s and has since spread throughout
the nation (Couch and Nelson 1985). It wasfirst found in Minnesota in 1987 in Lake Min-
netonka. Despite numerous measures to
prevent its proliferation and spread, EWM has
spread to 159 lakes as of summer 2004, most
being within the Twin Cities metro area. For a
detailed life history of Eurasian watermilfoil,
consult Smith and Barko (1990).
Eurasian watermilfoil grows rapidly,
typically forming extensive homogeneous sur-
face canopies that displace native macrophytes
(Madsen et al. 1991; Madsen 1997; Figure 3).
These dense canopies reduce sub-canopy light,oxygen, and pH (Carpenter and Lodge 1986;
Madsen 1997). This results in an inhospitable
foraging environment in the sub-canopy that
may confine predators to small open pockets
or bed edges (Killgore et al. 1989). Young
age-classes of largemouth bass are more effi-
cient feeders in native plant assemblages
compared with dense EWM canopies (Valley
and Bremigan 2002a).
Field studies suggest EWM depresses
fisheries only when it forms extensive, homo-
geneous beds throughout the littoral zone(Keast 1984; Lillie and Budd 1992; Engel
1995). Areas infested by large monospecific
beds of EWM tend to have less abundant fish
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A B
Figure 3. Schematic representation of a canopied macrophyte monoculture (A) and a diverse
macrophyte community (B). Figure from Madsen (1997).
and invertebrates than do areas with diverse
plants (Keast 1984; Cheruvelil et al. 2001).
Nevertheless, if EWM is part of a diverse
plant community or if it grows in patcheswhere open pockets permit fish movement,
fish populations generally are not nega-
tivelyaffected (Weaver et al. 1997; Olson et al.
1998; Valley and Bremigan 2002b). In fact,
EWM can be beneficial to fisheries if it occurs
in lakes that typically do not support much
growth of native submersed species (Engel
1995) because more fish and invertebrates are
found in areas with EWM than areas devoid of
SAV (Pratt and Smokorowski 2003).
Madsen (1998) investigated the corre-
lation between physical and chemicalcharacteristics of lakes and EWM dominance
(percent cover within the littoral zone) in 300
lakes across the US. He found lake trophic
status was the best predictor of EWM domi-
nance, with dominance highest in mesotrophic
( >10 g L-1 Total Phosphorus) to eutrophiclakes ( < 30 g L-1TP; Figure 4). Dominance,
then, decreased rapidly as lakes approached a
hypereutrophic condition (> 30 g L-1 TP).Madsen (1998) also documented that EWM
dominance was inversely proportional to cu-mulative native plant cover, suggesting the
presence of native plants reduces the probabil-
ity that EWM will dominate the littoral zone.
Nevertheless, EWM in-lake spreading can be
expected despite current control methods, with
some displacement of native plants (Madsen et
al. 1991); however, displacement of native
plants may not be permanent and EWM domi-
nance may decline over time (approximately
15 years; Nichols and Lathrop 1994).
In non-infested eutrophic lakes, native
macrophyte species such as coontail Cerato-phyllum demersum, naiadsNajas spp., Canada
waterweed Elodea canadensis, and northernwatermilfoil Myriophyllum sibiricum can formmonospecific canopies similar to those de-
scribed for EWM, and thus may affectfisheries in similar ways (Frodge et al. 1990).
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Figure 4. Relationship between Eurasian watermilfoil dominance (percent littoral cover) and totalphosphorus for 25 U.S. lakes. From Madsen (1998).
Curly-leaf pondweed Curly-leaf
pondweed (CLP) is a perennial, rooted, sub-
mersed vascular plant that was first noted in
Minnesota circa 1910 (Moyle and Hotchkiss
1945). Curly-leaf pondweed is currently
known to occur in 65 of the 87 Minnesota
counties (Exotic Species Program 1997).Curly-leaf pondweed can grow in a variety of
habitats, but its most prolific growth occurs in
shallow, soft-bottom areas (Nichols 1992).
Unlike most native plants, CLP remains green
and viable under thick ice and snow cover
(Wehrmeister and Stuckey 1978); therefore, it
is often the first plant to appear after ice-out.
By late spring, it can form dense mats that
may interfere with recreation and limit the
growth of native aquatic plants (Catling and
Dobson 1985). As a response to warm water
temperatures in mid-summer, CLP usually
senesces, leading to increases in concentra-
tions of phosphorus (Bouldan et al. 1994) and
algal blooms. Prior to senescence, CLP plants
form vegetative propagules called turions
(hardened stem tips) from which new plantssprout in the fall (Catling and Dobson 1985).
Published data on the relationship be-tween fish and CLP is sparse and thus present
understanding is mostly anecdotal. Duringspring, fish sampling in stands of CLP cap-
tures small fish, including juvenile game fish,
which suggests that this plant provides habitat
for these fish during winter and spring (J.
Laurer, MN DNR Fisheries Biologist, personal
communication; Miranda and Pugh 1997).
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Also low abundance, higher growth, and size-
structure of sunfish populations are common
in some lakes dominated by CLP (B. Ner-
bonne, R. Ramsell, MN DNR Fisheries
Biologists, personal communications). The
mechanisms underlying these observations
have not been studied, but limited spawninghabitat in spring, high predation upon juve-
niles after CLP senescence, or high lakeproductivity of infested lakes are potential fac-
tors.
At the end of spring when CLP
senesces or dies back, the decrease in
submersed vegetation often is followed byincreased algal blooms due to the release of
phosphorus into the water column. This
dramatic shift in habitat conditions may favor
disturbance-tolerant fish species such as black
bullhead, carp, fathead minnowsPimephalespromelas, and white sucker Catostomuscommersoni. Nuisance growth of CLP oftenoccurs in shallow, eutrophic basins (lake types
2 and 6) where native SAV has been lost due
to the loss of water clarity and carp.
DIRECT AND INDIRECT EFFECTS OF
AQUATIC PLANT MANAGEMENT
Given the ability of non-native plants
to alter fish habitat, not to mention impair rec-
reation and aesthetics, effort has been devotedto their management (Madsen 1997). Field
evaluations of the effects of aquatic plant
management (APM) on fish populations dem-
onstrate that the magnitude of effect is
dependent on the degree to which APM alters
total plant abundance and patchiness.
Little is known regarding the effects
of alternative control methods of CLP on fish
habitat and populations. Control methods can
target CLP because it grows under ice in win-
ter and is abundant early in the spring before
many native macrophytes. However, remov-ing CLP from shallow eutrophic lakes and
promoting native species represents a signifi-
cant challenge to managers. Removal of CLP
(or any other plant for that matter) without
replacement by native plants compromises fish
and wildlife habitat. More studies have been
conducted with Eurasian watermilfoil and hy-
drillaHydrilla verticillata. Below, we discusssome relevant findings.
Biological control with grass carpGrass carp Ctenopharyngodon idella elimi-nated all vegetation in Lake Conroe (8,100 ha
Texas reservoir), which was 44% hydrilla
prior to grass carp stocking (Bettoli et al.
1992). Elimination of all vegetation had pro-
found effects on the fish community shifting itfrom a community dominated by littoral spe-
cies such as largemouth bass and bluegill toone dominated by pelagic and river species
such as shad Dorosoma spp., catfish, andwhite bass (Bettoli et al. 1993). Grass carp are
indiscriminate herbivores and feed on native
plant species as well. Naturally, completeelimination of aquatic plants would not be de-
sirable for native Minnesota fish.
Whole-lake chemical controlIn LittleHorseshoe Lake (Crow Wing Co. MN; lake
type 4), the aquatic herbicide endothall and2,4-D, removed all SAV (approximately 50%
whole-lake cover prior to treatment) and 60%
of the floating-leaf vegetation in 1992
(Radomski et al. 1995). Despite a large reduc-
tion in SAV cover, changes in largemouth
bass, bluegill, and northern pike abundance
and growth could not be attributed to reduc-
tions in SAV abundance. Despite continued
absence of SAV in 1993 and 1994, fish growth
and abundance did not significantly change.
Growth indices of bluegill, largemouth bass,
and black crappie were significantly positivelycorrelated with summer air temperature. Al-
though slower growth generally occurred after
chemical treatment of SAV, observed growth
was neither consistently higher nor lower than
the predicted growth, based on summer air
temperature, after the elimination of SAV.
Water temperature (which varied greatly
among years) apparently had a more profound
influence on fish dynamics in Little Horseshoe
Lake than the abundance of SAV. This repre-
sents an example where a larger physical force
(i.e., climate) exerts a greater effect on the fishpopulations and overrides within-lake changes
in habitat. In addition, bluegill in Little
Horseshoe Lake may have altered their distri-
bution in the littoral area, perhaps by usingfloating-leaf and emergent vegetation cover
more than the denuded regions of the littoral
area. If bluegill were using shallow emergent
cover, foraging success of largemouth bass
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and northern pike may not have changed ap-
preciably with the elimination of SAV.
In two eutrophic Minnesota lakes
(Parkers and Zumbra, Carver Co.; lake type
4), chemical control of EWM by fluridone (>
10 ppb) greatly reduced the cover of vegeta-
tion with little recovery of native SAV (whole-lake extent was not documented; Pothoven et
al. 1999). Largemouth bass and bluegill tem-porarily experienced greater growth rates after
these lakes were treated, presumably because
of greater prey availability. However, most
effects did not last into the post-treatment
year, despite continuing declining vegetationin Zumbra Lake. Several nongame species
present prior to the fluridone treatments in
Zumbra Lake were not sampled during the
post-treatment year (Pothoven 1996).
Schneider (2000) evaluated the indi-rect effects of whole lake treatments of the
herbicide fluridone on game fish populations
in 11 mesotrophic Michigan lakes infested by
EWM. Concentration and contact time was
not evaluated, but less than 20 ppb was specu-
lated. Schneider (2000) observed large
changes in the plant community. In most
cases, fluridone initially removed nearly all
submersed vegetation. However, chara and
wild celery Vallisneria americana (commonpioneer species in mesotrophic lakes and
resilient to fluridone) rapidly recolonized de-nuded littoral areas and presumably increased
the habitat heterogeneity in these lakes. As a
result, bluegill and crappie size structure im-
proved significantly in many lakes after
treatment with fluridone. Sample sizes for
yellow perch, northern pike, and largemouth
bass were too small to determine whether
fluridone had a significant effect on these
populations.
In another Michigan study, eight
mesotrophic Michigan lakes were also treated
with low doses (5 7 ppb) of fluridone to se-
lectively remove EWM. These treatments did
not impact the fluridone-hardy, diverse plant
community that was present prior to the treat-
ment (Getsinger et al. 2001). However, it is
speculated that some fluridone-sensitive native
plant species (e.g., Elodea canadensis andCeratophyllum demersum) that were absent
before and after these treatments were lost as aresult of past treatments in these lakes (J.
Madsen, Mississippi State University, per-
sonal communication). Nevertheless, there
was no effect of the treatments on whole-lake
cover of SAV and no negative effects of thetreatment on fish and invertebrate populations
were detected (Cheruvelil et al. 2001; Valley
and Bremigan 2002b; Hanson 2001).
In a recent study in Minnesota, low-
dose fluridone applications (5 ppb target con-centration) were applied to three eutrophic
lakes infested with EWM (Crooked Lake,
Hennepin Co., Schutz Lake Carver Co. [lake
type 4] and Eagle Lake, Carver Co.[lake type
2]) in 2001. Preliminary analyses in these
lakes demonstrate large reductions in total
plant biomass and water clarity (W. Crowell
Minnesota Department of Natural Resources,
personal communication). Detailed hy-
droacoustic assessments were conducted in
Schutz Lake during 2002 and 2003 using
methods described by Valley et al. (in press).This analysis demonstrated a drastic decline in
the total abundance of submersed vegetation
within the littoral zone (Figure 5). Average
littoral biovolume (percent of the water col-
umn occupied by vegetation; excluding water
lilies) was 37% just prior to fluridone applica-
tion (early June 2002; Figure 5A). In August
2002, average littoral biovolume declined to
6.5% (Figure 5B). In June 2003, biovolume
was 3% (Figure 5C).
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Figure 5. Distribution of vegetation biovolume (percent of water column occupied by vegetation)in Schutz Lake just prior to 5 ppb fluridone applications during June 2002 (A), August
2002 (B), June 2003 (C), and August 2003 (D). Extensive floating-leaf vegetation was
present during both August sampling periods.
A
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white and yellowwaterlilies
Figure 5. Continued
B
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Figure 5. Continued
C
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White and Yellowwaterlilies
Figure 5. Continued
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In August 2003, littoral biovolume declined to
0.6% (Figure 5D). Despite low target concen-
trations designed to selectively remove EWM,
fluridone had negative indirect effects on total
plant cover because the removal of large
amounts of EWM from the water column fa-vored algal growth rather than recolonization
of barren areas by the few remaining nativespecies. Carp inhabited all three treatment
lakes and may have resuspended bare sedi-
ments, further exacerbating turbidity.
Mechanical control In theory, me-
chanical control of vegetation with harvestingcan be used to increase edge and vegetation
patchiness, thus potentially benefiting game
fish such as largemouth bass and bluegill
(Smith 1993; Trebitz et al. 1997). Cross et al.
(1992) examined the effect of harvesting mul-tiple plots (6% - 11% vegetation removal) in
two Minnesota lakes (Mary and Ida lakes,
Wright Co.; lake type 4) on largemouth bass,
bluegill, and northern pike populations. The
authors did not detect any significant effects of
the harvesting on the population metrics ex-
amined; however, they reported increased first
year growth of largemouth bass in the treat-
ment lakes. Cross et al. (1992) speculated the
manipulation was not large enough to effect
detectable changes in population metrics. In-
deed, Trebiz et al. (1997) determinedapproximately 20% - 40% of a fully vegetated
littoral zone would need to be removed in
patchy fashion to affect population growth
rates and size structure of bluegill and large-
mouth bass populations.
In Wisconsin, Olson et al. (1998) har-
vested many deep-cut channels perpendicular
to shore throughout littoral zones dominated
primarily by EWM in several lakes, with ap-
proximately 20% total vegetation removal for
each lake. Extensive pilot work was done
prior to this study to ensure the ability to de-tect significant effects if they did occur
(Carpenter et al. 1995; Trebitz et al. 1997).
Olson et al. (1998) documented increases in
growth for some age-classes of largemouthbass and bluegill; however, plants returned to
pretreatment densities after one growing sea-
son; consequently, this management strategy
requires harvesting each year.
Mechanical harvesting can uninten-
tionally kill juvenile fish, amphibians, and
turtles (Haller et al. 1980; Booms 1999). Hal-
ler et al. (1980) reported losses of 460 juvenile
fish per hectare of harvested hydrilla, and
Booms (1999) reported losses of 406 per hec-tare of harvested EWM. Apparently, fewer
fish are killed by deep-cutting compared with
mowing the surface (Unmuth et al. 1998).
Removing vegetation will have vary-
ing effects on fish populations depending on
the extent and distribution of manipulated ar-
eas. Harvesting and maintaining deep-cutchannels through offshore Eurasian watermil-
foil or curly-leaf pondweed beds could benefit
game fish populations because this type of
manipulation increases edge (Trebitz et al.
1997; Olson et al. 1998). However removingnearshore SAV and emergent vegetation (typi-
cally used as nursery areas) can have adverse
effects on fish diversity and game fish produc-
tion (Brian and Sarnecchia 1992; Jennings et
al. 1999; Radomski and Goeman 2001). Ra-
domski and Goeman (2001) documented
reduced biomass and mean size of northern
pike and sunfish populations in MN lakes with
little emergent or floating-leaf vegetation.
CONCLUSIONS
Given the diversity of Minnesota lakes
and the unique fish and habitat relationships
that are often found in these lakes, we cannot
responsibly prescribe a uniform level of SAV
abundance that is optimal for Minnesota fish
populations. Nevertheless, high coverage of
SAV within the littoral zone is highly impor-
tant to many fish species, especially in large or
deep lakes where SAV cover is limited.
With respect to non-native aquatic
plant species, our primary goal should be to
prevent (or more realistically, slow) theirspread among lakes. Within infested lakes,
probability of spread is high regardless of any
current control technique. For now, we must
accept that eradication of heavily infested
lakes is not a realistic management goal. Ac-
cordingly, actions should include those that
manage the nuisance to allow recreational ac-
cess without significant losses of fish habitat.
This recognizes that non-native plants do
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serve as habitat. In fact, there is little evidence
to suggest introductions of Eurasian watermil-
foil or curly-leaf pondweed into lakes will lead
to dramatic declines in fisheries.
When managing nuisance aquatic
plant species, managers should carefully con-
sider the lake type (i.e., Figure 1, Table 1) andassociated risks prior to making aquatic plant
management decisions. Large manipulationssuch as whole-lake herbicide treatments in
eutrophic lakes (e.g., lake types 2, 4, 6, and 8)
dominated, by mass, by the target species can
lead to large declines in total plant abundance
and water clarity. From a fisheries and wild-life perspective, overly abundant SAV is more
desirable than no SAV. For mesotrophic lakes
with diverse plant communities, whole-lake
treatments with selective herbicides may be
less risky. However, the cumulative long-termrisks of repeated treatments are unknown and
must be considered.
Vegetated or woody nearshore habi-
tats are especially important for fish
populations and any removal is a loss of fish
habitat. Accordingly, within any particular
lake, recreational access must be balanced
with the long-term sustainability and integrity
of the lake ecosystem. Past and ongoing land
use and SAV removal practices have perma-
nently altered many lakes within agricultural
areas of Minnesota and the Twin Cities metro-politan area. In others parts of the state, lakes
are threatened by rapid development.
The Minnesota DNR is charged with
protecting and managing Minnesotas aquatic
resources and associated fish communities for
their intrinsic values and long-term ecological,
commercial, and recreational benefits to the
people of Minnesota. Long-term sustainabil-
ity is the key objective and recreation andcommercial benefits must be regulated to pro-
tect long-term sustainability.
To achieve this end, a precautionarymanagement approach has been encouraged
not only in Minnesota (Welling 2001), but
globally as well (FAO 1996; Richards and
Maguire 1998; Restrepo et al. 1999; Auster2001). The precautionary approach advocates
action in advance of formal proof and shifts
the burden of proof to those challenging pre-
cautionary policy. A fundamental principle of
the precautionary approach requires proof that
proposed resource exploitation or manipula-
tion will not have significant adverse affects
on the resource (FAO 1996). For example, in
the context of APM, alterations to lakes in-
variably has some effect on the lakes fish
community and the onus should be on indi-
viduals or groups proposing manipulation tohabitats to prove that their activities will not
significantly harm fish populations (FAO
1996). It follows, that it is easier to prevent
harm than to later repair it.
Important elements of the precaution-
ary approach are the rules controlling SAV
removal, program and lake management plans,monitoring, enforcement, and evaluation. A
program management plan should include
mechanisms to monitor and control shoreline
alterations, and their aggregate impact on fish
habitat. Current limits for SAV control thatstate no more than 15% of the area less than
15 feet may be treated with chemicals or no
more than 50% of this area for mechanical
harvesting should not be viewed as some de-
sirable level of plant removal, but rather
define limits that when exceeded, compromise
fish habitat.
It is precautionary to have different
thresholds for different methods (e.g., more
regulation for APM where risks are larger).
Current APM thresholds may be safe for some
lakes, however, stricter thresholds may beneeded for soft water lakes and large, deep,
hard water lakes where SAV is more limited.
Also, a precautionary approach requires that
management be evaluated. Evaluations should
attempt to determine if management is robust
to uncertainty, prevents undesirable outcomes,
and monitors and addresses non-compliance.
Finally, socio-economic and political
considerations must be built into a precaution-
ary framework. Without significant attention
paid to these forces, precautionary manage-
ment will not succeed (Rosenberg 2002).Simplicity, transparency, and flexibility of
regulatory frameworks are key components of
successful precautionary management
(Rosenberg 2002). Building decision frame-works where key biological, socio-economic,
and political questions are raised and influence
the direction of policy decisions are currently
being used in Atlantic salmon management
(NASCO 2000). This decision analysis dem-
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onstrates a method of making precautionary
decisions and has implications for structuring
APM policy.
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ACKNOWLEDGMENTS
Many people offered helpful comments and suggestions on earlier drafts of this manu-
script and include: J. Anderson, W. Crowell, M. Drake, S. Enger, B. Nerbonne, R. Norrgard,
D. Pereira, D. Perleberg, C. Welling, and D. Wright.
Edited by: Paul J. Wingate, Fish and Wildlife Research Manager
Charles S. Anderson, Fisheries Research Supervisor